Can somebody explain this concept to me? of profesional gunners and non trained gunners?
Some aircraft have rear-gunners. The number of those can differ from 1 (Beaufighter, Stuka, BF110) over two (Me 410 - as it has two small single turrets on each side at the back) to I think 8 or so in the B17. The amount of gunners can be seen in the skill menu for gunners (at the top).
There is a perk called: Experienced gunners which is level one. You can upgrade this skill to two and further for the cost of 240 skill points and so on. It´s very expensive.
According you don´t have the 240 points (which is normal) this happens:
With just one rear-gunner there is no problem. You skill the points and he gets them.
Now you have an aircraft with two gunners. But you only have one experienced gunner. This means only 50% of the skills for your rear-gunners will come into effect. You see this with red (-1) numbers next to your skill-tree for the gunners.
With 3 gunners and the skill for experienced gunners on level one, you will only get 33,33% of the skill points to effect.
And so on.
If you have two experienced gunners you wont get a penalty on aircraft with one or two gunners. An aircraft with three gunner-seats however will only receive 66,66% of the points, a four seated bomber will get 50% of the perks into effect. If you want 100% on it then you need to have all the gunners experienced - which is insane in cost.